A power converter in which a plurality of power conversion circuits is connected in parallel is known (for example, JP 2009-118641A, JP 2007-116834A, and WO 2010/140228 A1). Such a power converter may be called a multi-phase converter. In a multi-phase converter, each power conversion circuit may simply be termed “a phase”, In such a power converter, the plurality of power conversion circuits is connected in parallel to distribute load thereon.
In a power converter of JP 2009-118641A, a plurality of bidirectional DC-DC converters (power conversion circuits) is connected in parallel. JP 2007-116834A and WO 2010/140228A1 describe multi-phase converters (power converters) in each of which converters of plural phases are connected in parallel. The multi-phase converter (power converter) of JP 2007-116834A determines a phase number of converters to be driven according to a magnitude of output load or a surrounding temperature. The multi-phase converter (power converter) of WO 2010/140228A1 determines a priority driving phase in an order of high heat diffusing performance. This multi-phase converter (power converter) prevents overheating of each converter by driving a power conversion circuit having a higher heat diffusing performance with priority. As an example of the heat diffusing performance, a temperature increasing rate of a switch portion of each converter is employed.